As typical examples of conventional communications systems which have employed a loop-type network system in which node devices are arranged in a loop, the road control system, railway control system, sewerage control system, airport control system, river control system and underground railway control system have been known.
In these systems, a plurality of node devices are provided at each place and interconnected to each other. Data is transferred to the node devices and each node device is controlled on the basis of the data received.
Take a road control system for example. In the road control system, a control centre is installed within each control area, and video cameras, emergency telephones, vehicle sensors and other apparatus used in road surveillance are located along the roads. Information obtained from these apparatus is gathered on the control centre, where it is displayed on surveillance monitors.
Meanwhile, electronic signboards are also installed along the roads in order to display various road information, and communication routes are established for the purpose of transmitting the information (data) obtained by the aforementioned apparatuses to the display devices.
The control centre also gathers information on accidents and traffic congestion from adjoining areas, and various judgments are made on the basis of the gathered information and information from the area itself. The results are displayed on the electronic signboards in the form of information on traffic congestion and various other types of guidance, thus helping to control the traffic.
FIG. 9 illustrates a road control system of this kind. The system comprises a plurality of node devices 10-1 through 10-8, and a communication route 40 which connects the node devices 19-1 through 10-8 so as to transmit and receive information therebetween.
Specifically, in FIG. 9, 10-1 is a control centre or a node device located in the control centre or each place, while 20-1 through 20-8 are local communication devices such as video cameras, emergency telephones, vehicle sensors, electronic signboards or monitors. 30 is a communication path control device (console), which controls the communication path in relation to the node devices 10-1 through 10-8.
In the system configured in this manner, the communication paths in relation to the node devices 10-1 through 10-8 are established through the control by the communication path control device 30. Meanwhile, 40 is an optical cable which constitutes the communication path, assuming the form of a loop which serves to transmit data between the local transmission devices by way of the plurality of node devices 19-1 through 10-8.
Recent years have witnessed the development of transmission systems of this sort which make use of the ATM (asynchronous transfer mode) exchange method and in which the communication path is established in the ATM exchange method.
FIG. 10 is a simplified version of FIG. 9, and will be used for the purpose of describing a path control system using the ATM exchange method.
In the conventional system configured as illustrated in FIG. 10, the communication path through which data (e.g., to display on a surveillance monitor screen images of traffic congestion picked up by a video camera) is transmitted between local communication devices 21-1 and 21-2 is connected at the node devices 15, 16, 17 by way of a single route of a bidirectional path with data transmission and data reception using the same VPI/VCI (virtual path/virtual channel) data.
However, there is a problem in establishing communication paths in a loop-type network as described above in that it is difficult to operate the communication paths if a system fault occurs.
For instance, as shown in FIG. 11, if a fault occurs in one of the node devices or in the optical cable which connects the node devices, the whole of the communication route associated with the faulty section is blocked. As a result, data transmission becomes impossible and the system ceases to operate. Thus, the occurrence of a fault on the communication route which forms the loop in a loop-type network is fatal for the operation of the system.
Moreover, when a diversion path is to be established onto a route which is unaffected by the fault, it is necessary to re-establish the communication paths to all the node devices, and after the fault has been repaired and service resumed, the communication paths to all the node devices must be checked. This means that the system will be inoperative for a long time, leading to a lowering of the control function. In the road control system designed to work night and day, this long inoperative time of the system will create severe disruption and cause panic in traffic.
As described above, the conventional network path establishing method is configured based on a path control method in which the communication path between local communication devices runs round the loop in such a manner that the transmission and reception paths follow the same route. With such a configuration, the occurrence of a fault will make data transmission impossible, and depending on the spot where fault occurs, it may be necessary to re-establish diversion routes for all the communication paths connected with that spot. In a system such as the road control system where transmission defaults at the outdoor location may occur at high probability, the communication path has to be disrupted in a long time at any times, and a great deal of work is required in order to re-establish a diversion route.